Phytases are a class of acid phosphatase enzymes that hydrolyze phosphates from phytate to produce free phosphate and inositol. Phytic acid (inositol hexakisphosphate), or its deprotonated form, phytate, is common in many animal feed components such as grains and legumes, and can represent a significant portion of the total phosphate content in these feeds. However, many livestock animals cannot efficiently digest phytic acid and are therefore unable to absorb the phosphate.
As a result, other forms of phosphate, such as rock phosphate or calcium phosphate, must be added to animal diets to provide this critical nutrient. Furthermore, phytic acid acts as an antinutrient in the diet, binding to proteins and chelating minerals such as iron, calcium and magnesium, which prevents their absorption. Since undigested phytic acid and excess inorganic phosphate can be excreted in the feces, they can act as a significant source of phosphate pollution in agricultural run-off. Phytase is commonly used in industrial processing and animal production. Inclusion of phytases in animal diets can alleviate the need to add inorganic phosphate, increasing the absorption of phosphate, proteins and minerals by the animal, and decreasing phosphate pollution from agricultural run-offs. When combined these effects can significantly increase the efficiency of animal growth and overall nutrition obtained from the feed they consume.
In industrial process, particularly fermentation processes, phytase is often used to hydrolyze phytate, releasing minerals and other nutrients into the fermentation, as well as enhancing starch degradation by enzymes that require cofactors sequestered by phytate (E. Khullar, J. K. Shetty, M. E. Tumbleson, V. Singh, “Use of Phytases in Ethanol Production from E-Mill Corn Processing,” Cereal Chem., 88(3):223-227, 2011, which is incorporated herein by reference as if fully set forth). It is also used industrially to reduce scaling that may be associated with phytate or phosphate build-up (sometimes referred to as “beer stone”), which often occurs in fermentation or related processes. In animal production and nutrition, one strategy for making phosphorus from phytate nutritionally available to monogastric animals is the addition of phytase to animal feeds (Jongbloed and Lenis, 1998; Onyango et al., 2005, both of which are incorporated herein by reference as if fully set forth). The use of phytase in the diets of poultry and swine has been shown to improve performance and phosphorus utilization (Baker, 2002; Nyannor et al., 2007 and 2009, both of which are incorporated herein by reference as if fully set forth). A number of phytase products are currently marketed for this use and include Natuphos™ (BASF), a phytase derived from Aspergillus niger, Ronozyme™ (DSM) a phytase derived from Peniophora lycii, and Quantum and Quantum Blue (AB Vista) phytases derived from Escherichia coli. The use of phytase in animal feeds allows a reduction in the amount of inorganic phosphorus added to animal feeds and has been reported to result in reductions in fecal phosphorus as high as 56% (Nahm, 2002; Sharpley et al., 1994; Wodzinski and Ullah, 1996, all of which are incorporated herein by reference as if fully set forth). While phytase use in animal feed and industrial processing is beneficial, one common challenge for using microbially or plant-produced phytases in animal feed diets is their inability to maintain full activity through the conditioning, extrusion, or pelleting processes commonly used to make feed pellets. Although some enzymes have been engineered to improve their thermal stability, most lose activity during pelleting, increasing their relative costs and decreasing the efficacy of the enzyme. Therefore, enzymes with further improvements in thermal stability are needed, particularly as feed manufacturers prefer to use higher-temperature pelleting processes.
It is well known in the art that many biomolecules can be rendered inactive through exposure to high temperatures. Because proteins are ubiquitous in nature, occurring in all kingdoms of life and being present in organisms as diverse as mesophiles to extreme thermophiles, they have an enormous range of thermal stabilities. Proteins that are characterized to have low thermal stability often progress through a molecular pathway wherein their structures increase in energy, increasing molecular vibration and movement, which overcomes intramolecular bonding forces and cause the protein to unfold. As unfolding occurs, structures within the protein are disordered, simultaneously exposing hydrophilic and hydrophobic regions and amino acids in the protein structure, and often leading to aggregation of the protein. For proteins that have low thermal stability, the unfolding process is often considerably faster than the refolding process, and in some cases may essentially be irreversible. Conversely, proteins that possess high degrees of thermal stability often have a greater degree of intramolecular bonding, which helps hold their structure together in the presence of increasing levels of thermal energy, as well as rapid rates of refolding, which can enhance a protein's ability to recover its activity when confronted by destabilizing thermal exposure. Given the broad range of thermal stabilities observed among different proteins, an opportunity exists to engineer less stable proteins to be more thermally stable. This is specifically relevant to phytases, which are often derived from mesophilic or less thermophilic organisms, and commonly struggle to maintain high levels of activity in animal feed pelleting processes, or industrial processes.
Another common challenge with producing heterologous proteins in plants, microbial cells, or other cellular production systems, is the risk that the heterologous protein poses as an allergen to humans. Any heterologously-expressed enzyme presents an allergenicity risk to those exposed to the protein through inhalation or ingestion. In order to reduce the allergenicity risk of the protein, particularly a plant-expressed protein that could be inadvertently consumed, it is desirable to engineer the phytase so that it has reduced stability when exposed to a gastric environment, an intestinal environment, or when exposed to pepsin. Reduced pepsin stability makes the protein safer as it would be readily digested in the human digestive tract if the plant material containing the engineered phytase was inadvertently ingested.